This invention relates to video subsystems for computers, such as personal computers, and more particularly to a more efficient arrangement of this nature having improved performance in relation to the control of arbitration and access to the memory of a subsystem of this nature
The invention is described herein in connection with a personal computer system. Systems of this nature have been described heretofore which include a microprocessor interconnected with various Input/Output (I/O) and storage elements by way of a bus. A personal computer may also include I/O slots for insertion of adapter cards by the user.
For example the U.S. Pat. No. 4,408,200 describes a personal computer system comprising apparatus and method for writing text characters for a raster scan video display operated in an all-points-addressable, or graphics, mode, and for thereafter reading characters. A processor writes a character to the display by selecting and loading into a graphics video display buffer a text character dot pattern retrieved from storage, and reads a character previously written by comparing a dot pattern retrieved from the display buffer with dot patterns retrieved from main storage. Provision is made for color display as well. The system illustrates a display or graphics adapter comprising a video subsystem, the adapter being inserted in an I/O slot.
An enhanced video subsystem is described in U.S. Pat. No. 4,580,135 having a raster scan display that includes a plurality of storage maps. The maps are addressable in either of two modes to provide addresses and display of video information by the system The system represents an enhanced graphics system organization, sometimes referred to as an Enhanced Graphics Adapter (EGA). The video memory is time shared by multiplexing access of a CPU and a CRT controller. Although speciiic cycle details are not set forth, the Enhanced Graphics Adapter (EGA) Card provides dedicated cycles for accessing the memory by both the CRT controller and the CPU. The EGA has two arbitration modes, a speed mode and a high speed mode. In the high speed mode, the CPU always has 1 cycle out of every 5 cycles in which it can access the video memory; in the low speed mode, the CPU has 3 out of 5 cycles to access the video memory.
Other art especially concerns arbitration in video subsystems. For example, the U.S. Pat. No. 4,511,965 concerns a system for resolving the contention between the central processing unit (CPU) and a cathode ray tube (CRT) controller in accessing a video memory array, or video random access memory (RAM), of a data processing system. A CPU access period is provided between successive CRT controller access periods Arbitration logic is included to provide CRT controller access priority when there is contention between the CPU and the CRT controller. No mention is made of adapting the arbitration scheme to the CRT controller's activity.
As another example, U.S. Pat. No. 4,117,469 describes a computer terminal having a CRT display, a microprocessor, and a random access memory which stores character codes for information to be displayed on the CRT screen and which serves as working storage for the processor. The display processing is interleaved with other chores of the microprocessor.
As still another example, U.S. Pat. No. 4,577,344 discloses a system for processing video signals representing sensed images The video signals are fed through a multiplexer to a vision processor and a display. The video signals are preprocessed and stored in the vision processor which has a shared memory. A computer controller has access to the shared memory via a common bus and controls operation of the vision processor in response to command signals from peripheral devices.
A primary objective of the present invention is to enable more efficient and rapid utilization of a video memory in a Video Graphics Array (VGA) by a video controller and central processing unit, the VGA operating in a variety of display modes classified according to resolution; color; and attributes, such as intensity level change, reverse video, underline characters and the like.